For many commercial applications images on an original color reversal films are electronically scanned, digitally stored in memory or recording media, and electronically modified if needed. The scanned and modified stored image is then re-written onto an output color reversal film (the "output film") by a film writer output device. In another application, output from computer generated images are written onto slides (included in the reference to "output film" in this application) for graphics presentations. The output device for these applications uses a precisely controlled light source for red, green, and blue exposure to record the image onto the output film. Typically the film writer light source is a cathode ray tube ("CRT"), although some film writers use arc lamps the output of which is controlled through light valves and a variety of other light sources and control methods are used in the trade. Examples of commercially available film writers include the FIRE 1000 film writer manufactured by Cymbolic Sciences Int., Richmond, British Columbia, Canada (for daylight balanced films), and the SATURN UR film writer manufactured by LVT Co., Rochester, N.Y., USA.
Currently, output devices record images onto existing color reversal films that were designed and optimized as normal camera picture taking films. Such films fall into two general classes, namely daylight exposure balanced film and tungsten balanced film. Thus, many film writers are designed to optimally write on one or both of those two classes. Although daylight and tungsten balanced films record the main attributes of the output image when exposed to the daylight balanced or tungsten balanced output, respectively, of a film writer, they fail to accommodate certain characteristics of the film writer output devices. In particular, the primary film writer limitation is its inability to accurately output light exposure over a broad enough intensity range to expose existing films from low density to high density. This is particularly true when higher exposures are used to produce lower film densities on the exposed and processed film. This exposure limitation can lead to "clipping" artifacts in the output image produced by film writers, such that very dark colors, very light colors, or both, which are present in the stored image, are not properly exposed and therefore not properly reproduced on the reversal film element.
Additionally, the limited characteristics of the film writer's output can lead to a failure to accurately reproduce densities on the output film such that they closely correspond with the densities on the original film. In a color system where the output film will have red, green and blue sensitive layers this is particularly critical. In particular, when an output film is used to receive the output of an electronic film writer, the density on each image portion on the processed output film should correspond as closely as possible with the density of the same image portion in the original. This requires that the density of each of the red, green and blue light for each image portion on the output film, should be as close as possible to those in the original. A greater deviation in even just one of these densities (red, green or blue light) anywhere throughout the range of densities that might be encountered in the original, can result in an increased failure of the output film to faithfully reproduce the colors of the original film.
Techniques for modifying what is known as the characteristic curve of a photographic element (or the D versus logE curve) are known. U.S. Pat. No. 3,849,138 describes a film designed with a larger latitude. U.S. Pat. No. 4,792,518 also describes varying the characteristic curve by controlling silver emulsions.
It would be desirable then to have a color reversal film which can be used with existing electronic film writers to more faithfully reproduce a broader range of image densities in the original film despite the limited range of light intensities which some existing electronic film writers can generate.